1. Field of the Invention
The present invention relates to a current limiting device. More specifically, the present invention relates to a current limiting device which is applied to a general AC circuit including all electric power systems (low voltage power systems to ultra-high voltage power systems), and is used to suppress over-current attributing to short-circuit accidents or in-rush current which may flow upon application of load.
2. Description of the Related Art
For instance, all power systems (low voltage power systems to ultra-high voltage power systems) have a current limiting device to suppress over-current attributing to short-circuit accidents or in-rush current which may flow upon application of load so as to reduce the interrupting capacity required for the circuit breaker. For example, as a current limiting device, there are a super-conductive current-limiting element utilizing the quench phenomenon of a super-conductor, or a thermal current-limiting element of a positive temperature characteristic (PTC) thermistor.
The super-conductive current-limiting element utilizing the quench phenomenon of a super conductor, such as a film type super-conductive current limiting element, is to increase the resistance instantaneously when over-current higher than the critical current occurs. That is, it is popularly employed as a high performance element because it is high in response, and of a self-operation type; that is, it is high in reliability. On the other hand, the thermal current-limiting device of the positive temperature characteristic (PTC) thermistor is formed with polymer material or barium titanate material. It generates heat when over-current is allowed to flow therein, so that it increases the resistance. Hence, it is also popularly employed being economical, and simple in construction.
The current-limiting device utilizing the super-conductive current-limiting element or the thermal current-limiting element is such that, as shown in FIG. 7, a current-limiting element S is series-connected between the system power source 1 of the power system and the load 2. For instance, in the case where a short-circuit accident a occurs to allow the flow of over-current, the resistance is increased because of the quench phenomenon. In the case of the thermal current-limiting element, heat is generated to increase the resistance. That is, the increase in resistance of the current-limiting element S suppress the over-current flowing in the electric power system. While the current-limiting element S performs its current-limiting function, a circuit breaker 3 is opened thereby to open the circuit of the electric power system.
In the above-described current limiting device, the current limiting element S is applied to the system whose linkage can be opened. However, the current limiting element can be applied to the system whose linkage cannot be opened. In the latter case, as shown in FIG. 8, a current limiting reactor 4 is connected in parallel to the current limiting element S. Normally, a current is allowed to flow in the current limiting element S. When a short-circuiting current flows in the element S; that is, when the element S is increased in resistance, the current is transferred to the current limiting reactor 4, and the switch S is operated to electrically disconnect the current limiting element S from the system, so that the system is ready for restoration.
A current limiting device utilizing the rectifying bridge circuit is as shown in FIG. 9. The AC terminals 15, 16 of the rectifying bridge circuit 14, which is formed with four diodes D1 through D4, are connected in series to the electric power system, and a DC reactor 19 is connected between the DC terminals 17 and 18. In the case where a high-speed response type current limiting effect is required, the current limiting device is suitable. Thus, it is popularly employed.
The current limiting device functions as follows when a short-circuiting accident occurs. That is, the AC current exceeds the DC current flowing in the DC reactor 19 due to the occurrence of the short-circuiting accident a, the pair of diodes D1 and D4, or D2 and D3 are stopped. If this is viewed from the AC terminals 15 and 16, during this period, the DC reactor 19 is inserted in series in the AC circuit, and the generation of a voltage across the DC reactor 19 suppresses the over-current flowing in the electric power system. While the rectifying bridge circuit 14 limits the current, the circuit breaker 3 is opened thereby to open the linkage of the electric power system.
By the way, the current-limiting device having the conventional super-conductive current-limiting element or thermal current-limiting element has the following problems.
First, the super-conductive current-limiting element is poor in thermal conduction in the direction of length, and it is considerably difficult to arrange the quench phenomenon occurrence characteristic in the direction of length. The thermal current-limiting element is considerably difficult to make the resistance-temperature characteristic completely uniform in the direction of length.
That is, in the case of those current limiting elements, only the element limited in length can be used in which heat generated partially is quickly thermal-diffused so that the whole is uniformly increased in temperature. Therefore, its rated voltage is of the order of 100V per an element. Hence, when it is applied, for instance, to electric power systems of 200V, 400V, 6600V or the like, at the time of current limitation, voltages corresponding to system voltages should be assigned to current-limiting elements. As a result, a plurality of current-limiting elements are connected in series.
However, if a plurality of current-limiting elements are series-connected, a common short-circuiting current flows in each of the elements. Therefore, if the elements are not uniform in characteristic, a part of the plurality of current-limiting elements is firstly increased in resistance. Consequently, heat-generating power concentrates at the element to thermally break the elements, or over-current concentrates as the elements to break the insulation thereof. This is a fatal problem.
Furthermore, the conventional current limiting device having the thermal current limiting element S, and the conventional current limiting device having the rectifying bridge circuit 14 have the following problems.
In the case that the thermal current limiting element S is the thermal current limiting element of a positive temperature characteristic (PTC) thermistor, the thermal current limiting element itself is simple in construction and economical. It functions in such a manner that, for instance at the time of a short-circuiting accident, the over-current generates heat, thus limiting the flow of current. However, the current limiting action has a delay of several tens of milli-seconds (msec), so that it is impossible to suppress the sudden current.
On the other hand, in the case of the rectifying bridge circuit 14, at the time of a short-circuit accident, the inductance of the DC reactor 19 positively limits the over-current. However, the DC current flowing in the DC reactor 19 increases every half cycle. Therefore, in order to expect the current limiting effect for a certain period of time, for instance 50 to 100 msec, both the cost and the loss are increased. Hence, in order to maintain the current limiting function for a relatively long period of time, for instance for more than 50 msec from the occurrence of a short-circuiting accident, it is necessary to provide a larger DC reactor. That is, the current limiting device using the rectifying bridge circuit is not economical.
In order to eliminate the above-described difficulties accompanying those devices, the following current limiting device has been proposed. That is, the thermal current limiting element S and the rectifying bridge circuit 14 are combined to mutually complement their characteristics. In other words, the device thus proposed is such that, at the time of occurrence of a short-circuiting accident a, the sudden current can be suppressed with simple means, and the current limiting function is maintained for a relatively long period of time.
The current limiting device described above is as shown in FIG. 10. The AC terminals 15 and 16 of a rectifying bridge circuit 14 are connected in series to the electric power system, and a DC reactor 19 is connected between the DC terminals 17 and 18 of the rectifying bridge circuit 14. On the AC side of the rectifying bridge circuit 14, a thermal current limiting element S is connected in series which generates heat in response to the flow of over-current thereby to increase its resistance.
In the current limiting device, the thermal current limiting element S and the rectifying bridge circuit 14 function to complement their characteristics. Accordingly, at the time of occurrence of a short-circuiting accident, the rectifying bridge circuit 14 positively suppresses the sudden current, thus providing a current limiting effect in high-speed response. Moreover, the thermal current limiting element S maintains the current limiting function for a relatively long period of time, thus providing a current limiting effect in low-speed response.
In the case of the current limiting device, the thermal current limiting element S is applied to the system whose linkage can be opened. However, the element S may be applied to the system whose linkage cannot be opened. In this case, as shown in FIG. 11, a current limiting reactor 20 is connected in parallel to a thermal current limiting elements. Normally, current is allowed to flow in the thermal current limiting elements. If a short-circuiting current flows to increase the resistance of the thermal current limiting element S, the current is transferred over to the current limiting reactor 10, and a switch 11 is operated to electrically disconnect the thermal current limiting element S from the system so that the latter S is restored.
The thermal current limiting element S combined with the rectifying bridge circuit 14 is limited in length. That is, the length must be such that heat generated at a part of the element is quickly diffused to increase the temperature of the element uniformly. Therefore, the rated voltage thereof is of the order of 100V per element. Hence, in the case where it is required to apply the thermal current limiting element to an electric power system of 200V, 400V, 600V, or the like, a plurality of thermal current limiting elements are connected in series, because they must share the voltage corresponding to the system voltage for the current limitation.
However, in the case where a plurality of thermal current limiting elements are connected in series, the short-circuiting current flows in each of the thermal current limiting elements. Therefore, if the thermal current limiting elements are not uniform in characteristic, some of the thermal current limiting elements become high in resistance. As a result, heat generating power is concentrated on those thermal current limiting elements, thus thermally breaking it, or the over-current is concentrated on them, thus breaking their insulation. This is a fatal problem in this case.
It is an object of the present invention to provide a current-winding device, in the case where a plurality of super-conductive current-limiting elements or thermal current-limiting elements are series-connected, even if those elements are not uniform in characteristic, which can prevent the breakage of those element by concentrating heat-generating power or over current concentrates at one of those elements.
The present invention provides a current-limiting device having a plurality of current-limiting elements connected in series and inserted in an AC circuit. The current-limiting element is a super-conductive current-limiting element which is increased in resistance by a super conductor quench phenomenon. Alternatively, the current-limiting element is a thermal current-limiting element, which generates heat by over-current and is increased in resistance by temperature rise. The current-limiting device have one of the following features.
(1) The current-limiting device has transformers in which the number of the transformers is equal to that of the current-limiting elements, primary windings of the transformers being connected in the same polarity, respectively, and secondary windings thereof connected in parallel to one another.
(2) The current-limiting device has windings in which the number of the windings is equal to that of the current-limiting element, and a common iron core to which the windings are magnetically coupled, wherein the windings are connected for the current-limiting elements in the same polarity, respectively.
(3) The current-limiting device has a plurality of blocks, each block comprising a plurality of primary windings, a secondary winding and a common iron core to which the primary windings and the secondary winding are magnetically coupled, wherein the primary windings of a plurality of the blocks are connected for the thermal current-limiting elements in the same polarity, and all the secondary windings of the blocks are connected in parallel to one another.
In the case of the super-conductive current-limiting element, in the steady state the current flows to the super-conductive current-limiting elements. However, when a short-circuiting accident occurs due to the fluctuation in characteristic, one or some of the plurality of super-conductive current-limiting elements are increased in resistance by the quench phenomenon. Accordingly, the short-circuiting current flows to the transformers which are in parallel with the elements thus quenched. Therefore, the current flowing to the elements is quickly decreased to prevent the generation of over-heat.
On the other hand, since the secondary windings which are provided for the elements in the same polarity, respectively, are parallel-connected to one another, the current flowing to the transformers quickly increases the currents of the elements which are not quenched yet, and forcibly quenches the elements. With the element which has been quenched, the abrupt decrease of element current occurs. The element which has not been quenched yet is quickly quenched because of the commutation of short-circuiting current. Hence, all the elements are quenched substantially at the same time; that is, the elements are uniformed in quench.
As was described above, the secondary windings provided respectively for the elements are connected in parallel to one another. Therefore, the terminal voltages (voltages across the primary windings of the transformers) of the elements are uniformed, and the difficulty is eliminated that over-current concentrates at part of the elements.
In the case of the thermal current-limiting element, when over-current flows, for instance, due to a short-circuiting accident, part of a plurality of the elements is increased in resistance because of the fluctuation in characteristic. When this phenomenon occurs, since the secondary windings provided respectively for the elements are connected in parallel to one another, current flows to the elements through the transformers so that the terminal voltages (voltages across the primary windings of the transformers) of the elements. Since the primary windings are provided in the same polarity, the direction of currents flowing through the transformers is such that, in the element which is increased in resistance, the current decreases, while in the element which is not increased in resistance, the current increases. Accordingly, the currents flowing through the transformers flow so that the elements are uniform in resistance.
A transformer on which windings are wound on one and the same iron core is equivalent to a multi-winding transformer. Therefore, similarly as in the above-described case, current flows through the windings so that the terminal voltages of the windings be uniform. And, since the primary windings are provided in the same polarity, the current flows so that the elements are uniform in resistance.
Further, the current-limiting device has another feature (4). That is, (4) the current-limiting device has windings in which the number of the windings is equal to that of the current-limiting element, wherein the windings is magnetically coupled to the current-limiting element, the windings is connected to the current limiting elements in the same polarity, respectively, and the combined inductance of the windings which are connected in series to one another is equal to or larger than a minimum value which is determined from an aimed current limiting value.
In the steady state, current flows in the super-conducting current limiting elements or the thermal current limiting elements. If an over-current flows by the occurrence of a short-circuiting accident, then since the elements are not uniform in characteristic, some of the elements are increased in resistance. As a result, the short-circuit current is transferred over to the windings connected to the defective elements.
In this case, the windings of the current limiting elements are connected to the current limiting elements, respectively, in such a manner that they are the same in polarity and are magnetically coupled to one another. Hence, because of the mutual induction of the windings, in the current limiting elements which are increased in resistance the element current flows in such a manner as to decrease, and in the thermal current limiting elements which are not increased in resistance the element current flows in such a manner as to increase. Therefore, with respect to the thermal current limiting elements which are increased in resistance, heat is prevented from being generated excessively, and with respect to the thermal current limiting elements which are not increased in resistance, the difficulty is prevented that the resistance is increased by the increase of the element current. Hence, even if the current limiting elements are not uniform in characteristic, they are uniformly increased in resistance.
In the case where the current limiting elements are applied to the system whose linkage cannot be opened, the device functions as follows: That is, when all the current limiting elements are increased in resistance, and the short-circuiting current is transferred to the windings, then those windings function as a current limiting reactor while being connected in series to each other.
Moreover, the present invention provides a current limiting device in which the AC terminals of a rectifying bridge circuit are connected in series to an electric power system, a DC reactor is connected between the DC terminals of the rectifying bridge circuit, and a plurality of thermal current limiting elements, which generate heat by over-current to increase the resistances thereof, are connected in series to the AC side of the rectifying bridge circuit. The current-limiting device has windings in which the number of the windings is equal to that of the current-limiting element, wherein the windings is magnetically coupled to the current-limiting element, the windings is connected to the current limiting elements in the same polarity, respectively, and the combined inductance of the windings which are connected in series to one another is equal to or larger than a minimum value which is determined from an aimed current limiting value.
According to the current limiting device, in the steady state, the current flows in the thermal current limiting elements. However, if a short-circuiting accident occurs, the over-current flows therein. If the thermal current limiting elements are not uniform in characteristic and some of the elements are increased in resistance, the short-circuiting current is transferred to the windings which are connected in parallel to the defective elements due to the resistance of the elements which have been thus increased.
In this case, the windings of the thermal current limiting elements are connected to the thermal current limiting elements in the same polarity, respectively, in such a manner that they are magnetically coupled to one another. Hence, because of the mutual induction of the windings, in the thermal current limiting elements which are increased in resistance the element current flows in such a manner as to decrease, and in the thermal current limiting elements which are not increased in resistance the element current flows in such a manner as to increase. Therefore, with respect to the thermal current limiting elements which are increased in resistance, heat is prevented from being generated excessively, and with respect to the thermal current limiting elements which are not increased in resistance, the difficulty is prevented that the resistance is increased by the increase of the element current. Hence, even if the thermal current limiting elements are not uniform in characteristic, they are uniformly increased in resistance.
In the case where the thermal current limiting elements are applied to the system whose linkage cannot be opened, the device functions as follows: That is, when all the thermal current limiting elements are increased in resistance, and the short-circuiting current is transferred to the windings, then those windings function as a current limiting reactor in such a manner that they are connected in series to each other.